Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g

Abstract

We report the first plausible optical electromagnetic counterpart to a (candidate) binary black hole merger. Detected by the Zwicky Transient Facility, the electromagnetic flare is consistent with expectations for a kicked binary black hole merger in the accretion disk of an active galactic nucleus [B. McKernan, K. E. S. Ford, I. Bartos et al., Astrophys. J. Lett. 884, L50 (2019)] and is unlikely [< Oð0.01%Þ)] due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernova and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as microlensing or a tidal disruption event, are ruled out or constrained to be < Oð0.1%Þ. If the flare is associated with S190521g, we find plausible values of total mass M BBH ∼ 100 M ⊙ , kick velocity v k ∼ 200 km s −1 at θ ∼ 60°in a disk with aspect ratio H=a ∼ 0.01 (i.e., disk height H at radius a) and gas density ρ ∼ 10 −10 g cm −3. The merger could have occurred at a disk migration trap (a ∼ 700r g ; r g ≡ GM SMBH =c 2 , where M SMBH is the mass of the active galactic nucleus supermassive black hole). The combination of parameters implies a significant spin for at least one of the black holes in S190521g. The timing of our spectroscopy prevents useful constraints on broad-line asymmetry due to an off-center flare. We predict a repeat flare in this source due to a reencountering with the disk in ∼1.6 yrðM SMBH =10 8 M ⊙ Þða=10 3 r g Þ 3=2 .

Properties and Astrophysical Implications of the 150 M⊙ Binary Black Hole Merger GW190521

“…Unlike the other possible scenarios, BBH mergers in an AGN disk may have an associated electromagnetic counterpart (an ultraviolet flare; McKernan et al 2019). No prompt electromagnetic counterpart has been reported for GW190521 (e.g., Barthelmy et al 2019;Casentini et al 2019;Doyle et al 2019aDoyle et al , 2019bLipunov et al 2019;Negoro et al 2019;Turpin et al 2019aTurpin et al , 2019bWatson et al 2019), but see Graham et al (2020), which appeared during the revision of this work, for a possible candidate optical counterpart.…”

Active Galactic Nuclei as Factories for Eccentric Black Hole Mergers

“…This is in stark contrast to the spherical cluster case, where only a few percent of 3-dimensional (3D) isotropic scatterings produce 3-body mergers 18 . Our results not only provide the first explanation for why GW190521 would naturally appear in the LIGO/Virgo frequency band with a measurable eccentricity if it did form in an AGN-disk 19 , but also opens up a rich new line of research on dynamically modeling the formation of binary black hole mergers in disk environments. Below we present our results by first deriving the merger probability and then the fraction of mergers that will manifest as eccentric sources in the LIGO/Virgo band.…”

“…Observationally, GW190521 is among the first gravitational wave sources with indications of an AGN-disk origin 5,19 . It is sensible to inquire whether its non-zero eccentricity 6, 7 is highly unlikely from an astrophysical perspective or if it could arise naturally as a distinct signature characteristic binary black hole merger during the interaction (3-body merger) with an eccentricity ∼ 0.5 in LIGO/Virgo.…”

“…A true milestone on the observational level is GW190521, which is the first gravitational wave source consistent with having a non-zero eccentricity 6,7 . Its relatively high-mass components, high misaligned spin and possible associated electromagnetic flare 19 suggest a possible AGN-disk origin 19 , but the additional non-zero eccentricity has been a major puzzle.…”

A Heavyweight Merger

“…Further hints may come from the recent identification of the possible electromagnetic counterpart of this merger. From the same region of the sky as GW190521, the Zwicky Transient Facility (ZTF) detected a flare emerging 35 days after the gravitational-wave signal (see Focus: Possible Flare from Black Hole Merger) [10]. This flare could have been produced by the gas in the black holes' environment, which gets heated by shock waves induced by the merger.…”